A jaw chuck for holding a work piece to the spindle of a rotary machine, such as a lathe, includes three or more jaw assemblies coupled to a chuck body for simultaneous reciprocating radial movement with respect to the chuck body to provide self-centering gripping of the workpiece. In a typical construction, each jaw assembly is attached to a master jaw disposed for radial movement within one of a number of radial jaw guides formed in the body. Each master jaw is coupled to a draw bar, disposed within the spindle of the machine, by a pivoting lever arm that actuates the master jaws by converting axial movement of the draw bar into the simultaneous reciprocating radial movement of the master jaws.
While the master jaws are rarely disassembled from the chuck body, the jaw assemblies or workpiece gripping jaws, are often removed from and reattached to the master jaws so that different jaw assemblies can be installed on the chuck so as to accommodate work pieces of different sizes and/or shapes. Each master jaw has a slot formed therein with retaining flanges extending along opposite edges of the slot. A top jaw is attached to the master jaw by means of two or more threaded fasteners extending through spaced-apart holes formed in the top jaw and into a single T-nut having two or more threaded holes or two or more one-hole T-nuts disposed in the master jaw slot. The fasteners are tightened into the T-nut(s) to draw the top jaw down into bearing contact with the master jaw. The respective surfaces of the master jaw and the top jaw that bear against each other are serrated to prevent sliding of the top jaw with respect to the master jaw.
This conventional method of attaching a top jaw to a master jaw has a number of disadvantages. Since the top jaw is attached to the master jaw with two or more threaded fasteners, the time required to change out the top jaws is greater than it would be if only one fastener were required to securely attach the top jaw to the master jaw. To attach a top jaw to the master jaw, each fastener must be placed through a hole formed in the top jaw and then aligned with and threaded into a T-nut disposed in the master jaw slot. Alternatively, the T-nuts can be started onto the ends of the fasteners protruding from the top jaw and then the loosely attached T-nuts are slid into the master jaw slots. The top jaw is then properly positioned with respect to the master jaw and both fasteners are tightened to secure the top jaw.
To remove the top jaw and install a new top jaw, both fasteners must be loosened, and the fasteners and T-nut(s) must be transferred to the next top jaw or the next top jaw must have its own dedicated fasteners and T-nut(s).
Also, to ensure rotational symmetry of the chuck and self centering gripping of the jaws, each top jaw must be positioned in the same location on its respective master jaw. For example, if the third serration of the top jaw is aligned with the first serration of the master jaw, then all top jaws should be aligned the same way. Aligning the top jaws can be cumbersome, because the serrated bearing surfaces makes it difficult to slide the top jaws with respect to the master jaws; both threaded fasteners must be sufficiently loose to permit adequate play between the top jaw and the master jaw.